The present invention is generally directed toward thread forming taps and, more particularly, to a new and improved thread forming tap for use in providing a locking thread form on the interior of a suitable aperture, bore, etc. In the art of thread forming, material is actually displaced, or cold rolled to form a thread within the aperture, bore, etc. Toward this end, the tap of the present invention is adapted to form a specialized thread within such a bore having a crest profile that will produce a self-locking thread.
All cold formed or rolled threads have a cup or xe2x80x9cUxe2x80x9d in the thread crest due to the nature of the thread forming process. This is shown in FIG. 1 (Prior Art). The cup is labeled xe2x80x9cUxe2x80x9d. The formation of each cup or xe2x80x9cUxe2x80x9d is the result of material being displaced to form the basic outline of a 60 degree included angle of a standard thread. A properly sized hole should result in a thread percentage of 65% to 75%. Tapping with too small of a pre-tap hole size produces a sharp thread form that results in excessive tapping torque, tap wear, and possible tap breakage. Tapping with too small of a pre-tap hole size results in a low thread percentage (i.e. 40%) and an after-tap minor diameter that is too big.
In the present invention, the cold forming tap produces a modified xe2x80x9cUxe2x80x9d, such that at least one leg of the xe2x80x9cUxe2x80x9d acts as a spring. The unique profile of the tap, described infra, changes the geometry and location of the xe2x80x9cUxe2x80x9d or cup on the thread crest. The unique profile does not interfere or change the basic thread pitch and major and minor diameters that could interfere with the assembly of a mating thread.
The present invention comprises an improved tap having a shank section, a body section extending longitudinally from the shank section, and a helical thread formed on the body section. The helical thread has radially outwardly converging flanks defining the thread root and thread crest. At least one flank includes an undercut angle formed between the pitch diameter and the root diameter of the tap. The undercut angle displaces or changes the location of the cup or xe2x80x9cUxe2x80x9d along the crest of the thread formed in a workpiece. In its new position, at least one leg of the xe2x80x9cUxe2x80x9d is now positioned to interfere with the mating male thread of a fastener that is threaded within the workpiece opening. The interfering leg member acts a spring that is displaced when the male threaded fastener enters the opening. The spring exerts a predetermined force against the fastener thereby increasing the torque required to drive the fastener into the threaded opening. This also increases the torque required to remove the fastener. As such, the fastener is better retained within the threaded opening and will not loosen over time.
If the fastener is removed and then reinserted, the spring leg of the xe2x80x9cUxe2x80x9d shaped crest will again exert a force against the fastener thread.
It is accordingly a general object of the present invention to provide a new and improved tap for providing a locking thread form in a suitable aperture, bore, etc.
It is a more particular object of the present invention to provide a tap for forming threads on a female threaded member wherein the threads are provided with an offset cup adapted to lockingly engage the crest of the threads of the associated male threaded member such as a screw, bolt or the like.
It is still another object of the present invention to provide a new and improved tap of the above-described type that may be utilized to form threads of optimum locking characteristics commensurate with the material of the workpieces in which the threads are to be formed, length of thread engagement, installation torque requirements and/or reversal torque requirements.